For example, mining wastewater contains pyrite (FeS2), and, when this pyrite is oxidized, SO42− is generated. In order to neutralize mining wastewater, inexpensive Ca(OH)2 is used. Therefore, mining wastewater contains a rich amount of Ca2+ and SO42−.
In addition, it is known that brine water, sewage water, and industrial wastewater also contain a rich amount of Ca2+ and SO42−. In addition, in cooling towers, heat exchange occurs between high-temperature exhaust gas discharged from boilers and the like and cooling water. Since some of cooling water turns into vapor due to this heat exchange, ions are concentrated in cooling water. Therefore, cooling water discharged from cooling towers (blow-down water) is in a state in which the ion concentrations of Ca2+, SO42−, and the like are high.
Water containing a large amount of these ions is subjected to a desalination treatment. As a concentration device for carrying out the desalination treatment, for example, reverse osmosis membrane devices, nanofiltration membrane devices, ion-exchange membrane devices, and the like are known.
However, in a case in which the desalination treatment is carried out using the above-described devices, if a high concentration of a cation (for example, a calcium ion (Ca2+)) and an anion (for example, a sulfate ion (SO42−)) concentrate on membrane surfaces when fresh water is obtained, there are cases in which the concentration of the ions exceeds the solubility limit of calcium sulfate (gypsum (CaSO4)) which is a poorly-soluble mineral salt, and there is a problem in that the ions are precipitated on membrane surfaces as deposits and the permeation rate (flux) of permeated water decreases.
Therefore, in the related art, as monitoring methods for reverse osmosis membranes, for example, a method in which the generation of the crystals of mineral salts is detected by means of visual determination using cells for monitoring reverse osmosis membranes in reverse osmosis membrane devices has been proposed (PTL 1).
In addition, a method in which at least part of concentrated water from a water conversion device is permeated through a separation membrane for monitoring and the precipitation of deposits included in the concentrated water on the membrane surfaces of the separation membrane for monitoring is monitored using pressure meters provided before and after the separation membrane for monitoring has been proposed (PTL 2). This proposal enables the early monitoring of the precipitation of deposits on the membrane surfaces of filtration membranes caused by the concentration of raw water (seawater) and the efficient suppression of the precipitation of deposits on the membrane surfaces of filtration membranes in water conversion devices.
In addition, PTL 2 has also proposed the supply of an alkaline medicine to concentrated water being supplied to the separation membrane for monitoring in order to promote the precipitation of deposits.
Furthermore, in the technical manuals of reverse osmosis membrane devices, there are cases in which the membrane surfaces of RO elements are contaminated by mineral scale, microbes, colloidal particles, and organic substances during the operation of reverse osmosis membrane devices, sediment is deposited on the membrane surfaces, and finally, it becomes impossible to obtain predetermined permeated water flow rates and predetermined desalination percentages. There has been a proposal in which elements need to be washed in a case in which the standardized permeated water flow rate decreases by 10% or more, a case in which the standardized saline component flow rate increases by 10% or more, or a case in which the standardized pressure difference (the pressure on the supplied water side−the pressure on the concentrated water side) increases by 15% or more during operation for the first 48 hours from the system initiation (NPL 1).